CHAPTER 7
Alkenes: Reactions and Synthesis
Preview of Reactions
Section 7.1 Preparation of Alkenes: A
Preview of Elimination Reactions
• Two most common ways to prepare alkenes
• Dehydrohalogenation
• Dehydration
• Ex: 2-bromopentane reacting w/ KOH in EtOH
KOH
EtOH
• Ex: 1-Hexanol reacting w/ aqueous sulfuric acid in THF
H2SO4
THF
Section 7.2 Addition of Halogens to
Alkenes
Cl2
Halogens react exclusively with C-C double bonds (not carbonyls, etc.)
Cl2
Reactions Involving Cycloalkenes
• Halogens can react with cycloalkenes to form trans-1,2-
dihaloalkanes
• Ex: cyclohexene reacting with Br2
Br2
anti-addition ONLY is observed during halogenation of cycloalkenes
resulting in the trans product exclusively
Section 7.3 Halohydrin Formation
• After formation of the bromonium ion the only nucleophile
present in the previous example is bromide
• What happens when additional nucleophiles are present?
No syn addition
observed
NBS
H2O Acetone
92%
Examples
NBS
NaN3 Acetone
?
What product would exhibit Markovnikov addition?
1 equiv. Br2
?
Section 7.4 Addition to Water to Alkenes:
Oxymercuration
• Hydration of alkenes is possible leading to the formation
of alcohols (introduction of –OH functional group)
• Most straightforward method involves the use of an
aqueous solution of a strong acid
H3PO4
250 C
Oxymercuration Procedure
• Oxymercuration utilizes mercury (II) acetate in an
water/THF solvent to convert hydrate alkenes to produce
alcohols
• The intermediate that is initially formed is then reduced with NaBH4
(sodium borohydride) to yield the alcohol
Section 7.5 Addition of Water to Alkenes:
Hydroboration
• Represents an additional method of converting alkene
into alcohol
• Complementary to oxymercuration because the hydroxy group is
introduced in a non-Markovnikov fashion
1. Hg(OAc)2, H2O
2. NaBH4
1. BH3, THF
2. H2O2, OH-
Stoichiometry of Hydroboration Step
• Borane (BH3) contains three reactive hydrogens and will
therefore react with three equivalents of alkene
• Ex: Cyclohexene reacting with BH3/THF
Mechanism
• Addition of borane occurs in a single step and occurs in a
syn fashion. No pure carbocation is formed, only a partial
positively charged carbon:
• Borane adds to the less substituted carbon for two basic
reasons
• Stability of carbocation
• Steric interactions
Steric Hindrance
2 Carbocation
+
1 Carbocation
+
Example
• Notice that the second step of the reaction involves peroxide which
oxidizes the borane intermediate to the desire alcohol product
1. BH3, THF
1. Hg(OAc)2, H2O
2. H2O2, OH-
2. NaBH4
Section 7.7 Reduction of Alkenes:
Hydrogenation
• Hydrogenation reactions are performed using a heterogeneous
catalyst (typically Pt, Pd/C, or PtO2)
• Involves syn-addition of H2 across the double bond of an
alkene
• Highly influenced by steric interactions
• Will add to the least sterically hindered side of a double bond which
has implications for stereochemistry in cycloalkenes
Examples
H2
Pd/C or PtO2
EtOH
H2
Pd/C or PtO2
EtOH
syn product observed
Section 7.8 Oxidation of Alkenes:
Epoxidation and Hydroxylation
• Hydroxylation involves the addition of two –OH groups
across the double bond of an alkene
1. OsO4
2. NaHSO3
A 1,2-Diol Product
(Glycol)
1. OsO4
2. K3Fe(CN)6
80%
Stereochemistry of Hydroxylation
Reactions
• Where stereochemistry must be considered
(cycloalkenes) hydroxylation using OsO4 occurs via syn
addition yielding the cis product exclusively
1. OsO4
2. NaHSO3
Epoxidations Using m-CPBA
• Epoxidations involve the creation of an oxygen containing
three-membered ring and can be used as a
complimentary way creating 1,2-diols
epoxide functional group
meta-Chloroperoxybenzoic acid
m-CPBA
CH2Cl2 (solvent)
syn-addition is observed
Epoxides from 1,2-Halohydrins
• Many times performed in two steps
• Conversion of alkene to halohydrin
• Conversion of halohydrin to epoxide
NaOH
H3O+
trans-substituted product
observed only
Section 7.9 Oxidation of Alkenes:
Cleavage to Carbonyl Compounds
• Products from ozonolysis depend on the degree of substitution in the
reactants
Tetrasubstituted alkenes yield two products containing the ketone functional
group
Disubstituted alkenes yield two products containing the aldehyde functional
group